THE National Roads Authority (NRA) wrote off more than €3.3m in unpaid M50 tolls last year. Of this sum, 57pc were incurred by Irish-registered drivers that the agency has simply given up pursuing.

The chairman of the Dail's Public Accounts Committee (PAC), Carlow-Kilkenny TD John McGuinness, has questioned this "high sum" and said the write-off called into question the NRA's ability to collect unpaid tolls.

He said the PAC would be calling the agency back before it to explain why so many Irish motorists were able to avoid the toll and subsequent fines.

Barrier-free tolling was introduced on the M50 in 2008. It nets the NRA approximately €100m a year.

There is a 96pc compliance rate among motorists who use the motorway, but the €3.3m write-off indicates that the remaining 4pc are proving difficult to pursue.

While most of those who avoided the tolls were drivers from the Republic, 28pc were driving northern-registered cars, while a further 15pc had plates from other jurisdictions.

Appearing before the PAC last month, NRA boss Fred Barry said the agency's enforcement action was carried out by a firm of solicitors.

He said those who haven't paid are prosecuted each month in district courts but conceded: "Some people simply escape."

He told the TDs that overseas drivers who do not pay were "very hard to pursue" and promised to provide figures to the committee for how much the NRA had to write off.

Deputy McGuinness received a letter yesterday, outlining the sum written off, and commented: "They (the NRA) issued 15,000 legal demand letters in their attempts to collect it and 500 civil summonses per month – and they still had to write off €3.3m.

"Their ability to collect is being called into question."

The Fianna Fail TD added: "They need to come before us and explain why 57pc of Irish vehicles did not pay. I just feel that the figure is really high."